Trophic transfer of synthetic polymers in a tropical estuarine food web.

The occurrence and distribution of plastics in tropical aquatic ecosystems are of increasing environmental concern, yet quantitative evidence linking tissue-associated plastic burdens with food-web structure remains limited in tropical estuarine systems. This study quantified microplastic and operationally defined submicron plastic-particle burdens across the Cross River estuary food web using stable isotope analysis (δ¹³C and δ¹⁵N) and Raman spectroscopy across five stations over a five-year monitoring period (2021-2025). Plastic contamination occurred across environmental matrices, with dry-season concentrations reaching 5.8 ± 0.6 items L⁻¹ in surface water and 45.4 ± 12.1 items kg⁻¹ in sediment. Polyethylene and polypropylene were the predominant polymers in biological tissues. Generalized Linear Mixed-Effects Models (GLMMs) showed that consumer trophic position and season were significant predictors of tissue-associated microplastic burden. Microplastic burden increased with trophic position (βTP = 1.082, p < 0.001, R²m = 0.61), while the dry season was associated with higher burdens (βSeason[Dry] = 0.415, p < 0.001). The corresponding trophic magnification factor was 12.08 (95% CI: 8.15-17.91). Operationally defined submicron plastic-particle burden was similarly positively associated with baseline-corrected δ¹⁵N enrichment (βΔδ¹⁵N = 0.453, p < 0.001, R²m = 0.55) and was higher during the dry season (βSeason[Dry] = 0.328, p < 0.001). Station and year accounted for additional spatial and interannual variability, with conditional R² values of 0.68-0.74. The absence of significant trophic-position × season interactions indicates that the positive trophic associations were broadly consistent across hydrological periods. These findings indicate trophically structured tissue-associated plastic contamination in the Cross River estuary while indicating that the observed statistical relationships do not by themselves establish a definitive dietary transfer mechanism. The occurrence of tissue-associated plastic particles in commercially important fish highlights a potential dietary exposure pathway and supports further investigation of food-safety implications and localized plastic-waste management.